1. Field of the Invention
This invention generally relates to the deposition of metal layers on a substrate and more particularly to CVD methods for selectively converting silicon to a refractory metal and to MOS semiconductor devices formed thereby.
2. Description of the Prior Art
In the manufacture of semiconductor devices there is a continuing desire to increase device density on the semiconductor chips in order to increase the resulting operating speeds of the device. The tight packing of devices on the chips places a premium on manufacturing techniques to meet the high standards of reliability necessary for such devices. In the manufacture of MOS devices, there has been considerable investigation of the deposition of refractory metals, such as tungsten and molybdenum. Refractory metals have excellent diffusion barrier properties and low contact resistance with silicon.
Presently, selective deposition of tungsten is performed by silicon and hydrogen reduction of tungsten hexafluoride gas. It is well known that tungsten may be deposited by only silicon reduction of tungsten hexafluoride according to the following equation: EQU 2WF.sub.6 +3Si.fwdarw.2W+3SiF.sub.4.uparw.
The prior art has taught, however, that silicon reduction is self-limiting and results in encroachment problems. The addition of hydrogen reduction is used to deposit tungsten in the desired thickness. The hydrogen reduction method, however, also results in severe encroachment problems. In addition, the selectivity in depositing tungsten strictly depends on the predeposition condition of the wafer, deposition chamber design, vacuum capability, the method of heating the substrate, and elaborate bake and purge cycles. Poor reproduceability, slow growth rates at turret temperatures below 400.degree. C. (less than 10 nm per minute), and selectivity breakdown are the major drawbacks of this process. In addition, HF liberated in the hydrogen reduction process etches recess oxide isolation areas of the devices, resulting in penetration of tungsten hexafluoride along the oxide edges. Also, the density of exposed silicon determines the selectivity, therefore reducing the selectivity of the process.
Several prior art methods have been suggested to overcome the above problems in tungsten deposition. In the commonly assigned U.S. patent application Ser. No. 044,262, filed Apr. 30, 1987, the substrate, after being exposed to WF.sub.6 gas to convert a limited thickness of silicon, a subsequent layer of silicon is deposited using a plasma deposition and the wafer is then again exposed to the WF.sub.6 gas. These steps are repeated until the desired thickness of silicon is converted to tungsten. In U.S. Pat. No. 4,595,608, the substrate is periodically removed from the CVD deposition environment and moisture is applied to the areas on which tungsten is not to be deposited. The substrate is then redisposed in the CVD environment. The process uses tungsten and hydrogen reduction.